The present invention relates to a rapidly rotatable optical mirror, and an optical scanner and a laser machining apparatus employing the improved mirror.
The prior art optical mirror will be described hereinafter with reference to the accompanying drawings.
FIGS. 12A and 12B show the structure of conventional lightweight optical mirror 120.
Optical mirror 120, as shown in FIG. 12A, comprises reflecting surface 121 and holder 128 which a motor shaft (not shown) is attached thereto. Holder 128 further comprises semi-circular cross section groove 122 for the motor shaft, and screw holes 123.
FIG. 12B shows the structure of the mirror seen from the rear surface. As shown in FIG. 2B, reflecting surface 121 has on its rear surface:
(1) mirror support beam 124 extending from holder 128;
(2) plural strengthening ribs 125 extending from the both sides of beam 124 toward the rim of the rear surface of reflecting surface 121; and
(3) peripheral ribs 126 that are disposed close to holder 128 and extended along the rim of the rear surface of reflecting surface 121.
The optical mirror structured above operates in a manner, which will be described hereinafter. Optical mirror 120 (FIGS. 12A, 12B) is attached directly to the rotary shaft of the motor (not shown), and used for a galvanometer scanner in which the rotation angle of the motor defines a reflecting direction of light.
Laser beam and illumination light are reflected by mirror surface 121. The shape and area of the reflected light depend on the shape of incident light and the rotation angle of the optical mirror.
To attach optical mirror 120 to the motor shaft, the motor shaft is fitted in semi-circular groove 122 and held with a retaining ring (not shown) having also a semi-circular groove, then secured by screws at screw holes 123. Diameters both of groove 122 and the groove of the retaining ring are generally sized to be a few micrometers bigger than that of the motor shaft. However, the perimeter of the motor shaft measures bigger than the perimeter of roughly semi-cylindrical shape formed from facing each semi-circular portion of groove 122 and the retaining ring""s groove. Therefore, fastening the screws to secure the optical mirror to the motor shaft inconveniently applies a stress to screw holes 123 vertically with respect to the reflecting surface 121.
Optical mirror 120 is required to keep enough rigidity against a distortion occurred between reflecting surface 121 and holder 128 while the motor is rotating. For keeping enough rigidity, mirror support beam 124, plural ribs 125, and peripheral ribs 126 close to holder 128 are formed on the rear surface of reflecting surface 121. In addition, as shown in FIGS. 12A and 12B, reflecting surface 121 of optical mirror 120 and holder 128 are formed in one piece.
Mirror support beam 124 functions as an absorber of the vibrations created in the axial direction of the motor shaft while the motor is rotating. Further, ribs 125 and 126 make a large contribution to minimize the fluttering of the mirror when rotating.
With the structure described above, however, a distortion occurs locally in the mirror surface when fastening the screws. When an optical scanner with such optical mirror is used for controlling the traveling path of laser beam in laser machining, flaws have often been detected outside the machined main hole in a workpiece.
FIG. 13 shows a distortion in the mirror surface when the motor shaft is attached and secured by screws to the conventional optical mirror, indicating distorted area by the curves.
It is apparent from FIG. 13 that the distortion which occurs at the screw holes disposed on the both sides of groove 122 is, through the peripheral ribs disposed on the rim of the mirror surface, carried to the mirror surface near the holder.
According to an amount of distortion measured by an interferometer, in the optical mirror made of a material containing beryllium for weight reduction, the Peak-Valley (P-V) value of the precision of the mirror surface measures no less than 4 xcexcm. This amount of distortion is compatible to the optical path difference of approximately one-half of the wavelength (approx. 10 xcexcm) of a carbon dioxide laser having relatively long wavelength. Generally, {fraction (1/20)}th of the wavelength of laser is defined to be optically aberration-free value (that is, approx. 0.5 xcexcm for a carbon dioxide laser.) The P-V value in FIG. 13, however, shows as much as about 10 times the aberration-free value for the carbon dioxide laser.
Referring to FIG. 14, now will be described a two-dimensional optical scanner using the conventional optical mirror.
The conventional two-dimensional scanner, as shown in FIG. 14, comprises two sets of galvano-mirrors 140A, 140B and position control unit 148. In FIG. 14, galvano-mirror 140A further comprises motor 143A having motor shaft 142A, and optical mirror 141A attached to motor shaft 142A. Motor 143A contains a position sensor (not shown) for position control. An output signal from the position sensor is fed into position control unit 148 for adjusting the position of the optical mirror. The explanation for galvano-mirror 140B will be omitted because the mirror has the same structure as mirror 140A described above. Hereinafter, depending on the parts constituting mirror 140A or 140B, either letter xe2x80x9cAxe2x80x9d or xe2x80x9cBxe2x80x9d is appended to the corresponding parts number.
Optical mirror 141A of galvano-mirror 140A, as shown in FIG. 14, horizontally rotates about motor shaft 142A, while mirror 141B of galvano-mirror 140B vertically rotates about motor shaft 142B.
The optical scanner structured above operates in a manner, which will be described hereinafter. Optical mirror 141A reflects laser beam 145 shown in FIG. 14 to direct an intended position on optical mirror 141B. In response to the reflection, the position sensor, which is built in motor 143A of galvano-mirror 140A, detects the orientation of mirror 141A. Getting the signal back from the position sensor, position control unit 148 adjusts the reflecting direction.
Similarly, in response to the light incident on mirror 140B, the position sensor, which is built in motor 143B, detects the orientation of mirror 141B. Getting the signal back from the position sensor, position control unit 148 adjusts the reflecting direction.
However, with the two-dimensional scanner employing mirrors 141A and 141B that have the conventional structure, the aimed surface cannot be radiated with the laser beam reflected from mirrors 141A and 141B due to a bad distortion.
FIG. 15 shows an optical system of the laser machining apparatus equipped with the optical scanner illustrated in FIG. 14. In FIG. 15, the conventional laser machining apparatus comprises:
a) laser oscillator 151 that produces a laser beam;
b) collimator 152 collimating the output laser beam from laser oscillator 151;
c) mask changer 153 masking the collimated laser beam;
d) reflecting mirror 154 reflecting the laser beam passed through mask changer 153;
e) two-dimensional optical scanner 155 scanning the incident laser beam through reflecting mirror 154;
f) scanning lens 156 projecting the incident laser beam through optical scanner 155; and
g) two-dimensional machining table 158 for mounting workpiece 157 to be machined with the projected laser beam. (Workpiece 157 is an object to be machined on machining table 158.)
The laser machining apparatus structured above operates in a manner, which will be described hereinafter. Laser oscillator 151 produces laser beam. After changed the beam diameter by Collimator 152, the laser beam is irradiated over the mask placed on mask changer 153. A portion of the laser beam, which passes through the mask, is launched into optical scanner 155 for controlling the scanning direction. Then scanning lens 156 projects the shape of the mask on workpiece 157 sitting on the two-dimensional machining table. Workpiece 157 is machined according to the projected mask shape.
FIG. 16 shows the strength distribution of laser spots, comparing with each other the strength at some spots in the entire scan area. If there is any distortion in the optical mirror, the strength distribution of laser spots varies depending on the position of the scan area. FIG. 16 shows the state of the distribution schematically. The strength distribution of laser spots is obtained by the position-by-position calculation of the scan area, using the machining optical system shown in FIG. 15 and, the data measured by an interferometer, which indicates the distortion of the mirror. FIG. 16 shows the calculated strength distribution of laser spots, comparing the strength with each other at nine spots in the scan area.
At central spot 161 of the scan area, as shown in FIG. 16, main beam 161A for machining maintains its diameter""s shape being circular (i.e., symmetric.) However, for example, at peripheral spot 162 of the scan area, main beam 162A for machining has no longer the symmetry in its shape. Furthermore, some beams with asymmetric beam diameter, for example, 162B, 162C, and 162D, are observed outside the main beam 162A. Each asymmetric beam has appreciable beam strength. The fact has an adversely affect in machining a workpiece made of resin with a relatively low work threshold. That is, at central spot 161 where main beam 161A maintains its beam diameter being asymmetric, the machined hole on a workpiece maintains its shape being circular (i.e., symmetric.) However, for example, at peripheral spot 162, the machined hole on the workpiece undergoes a distortion due to an asymmetric shape of beam diameter. Besides, some asymmetric beams existed outside the main beam make unwanted holes near the machined main hole in a workpiece. Such workpiece has been treated as a serious nonconforming piece due to the flaws near the machined main hole.
The present invention addresses the problems above. This provides an optical mirror with a structure minimizing a distortion that occurs in the mirror surface due to the stress from fastening screws when the optical mirror is attached to the motor shaft. It is an object that a laser machining apparatus with the mirror offers a consistent machining quality throughout the scan area.
The optical mirror of the present invention comprises a reflecting surface having optical characteristics, a holder to attach the mirror to other member, and a plurality of ribs disposed on the rear of the reflecting surface. The mirror also has slits in the ribs peripherally disposed close to the holder.
In the optical mirror that is attached to the motor shaft and rotates, the mirror comprises:
(1) a reflecting surface having optical characteristics;
(2) a holder to attach the mirror to motor shaft;
(3) a mirror support beam centered across the rear surface; and
(4) ribs extending from the support beam toward the rim of the rear surface.
The mirror is also structured so that the motor shaft-to-be-attached surface, or attachment surface, of the holder is held almost vertically with respect to the reflecting surface, or in other words, such that the attachment surface is substantially perpendicular with respect to the reflection surface.